The gas block helps direct the gas from barrels and fires bullets from the port, into the block, and down to the gas tube. It moves the bolt carrier group to allow the auto-cycling for the next round from the chamber. The standard diameter for a AR 15 gas block
size is 0.750” and for the lightweight and thinner barrel carried over the original M16A1 design, then choose the AR 15 Gas Block with a diameter of 0.625”. Similarly, for heavy barrels, it may need a 0.936” Gas block. You can also upgrade your AR 15 Gas Block to control the overpressure in the gas system better.